A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and other specialized tools to measure CD, overlay (the accuracy of alignment of two layers in a device) and defocus of the lithographic apparatus. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
The targets used by conventional scatterometers are relatively large, e.g. 40 μm by 40 μm gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of the target as it can be regarded as infinite. However, in order to reduce the size of the targets, e.g. to 10 μm by 10 μm or less, so they can for example be positioned in amongst product features, rather than in the scribe lane, metrology has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Typically such targets are measured using dark-field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed.
Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple targets can be measured in one image.
In the known metrology technique, overlay measurement results are obtained by measuring the target twice under certain conditions, while either rotating the target or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st diffraction order intensities. Comparing these intensities for a given grating provides a measurement of asymmetry in the grating.
Advanced lithographic processes require high-quality CD metrology for yield improvement and control. This technique is useful in advanced technology nodes because of its ability to non-destructively and rapidly retrieve accurate CD information by theoretical modeling of the spectral response under measurement. Optical CD metrology requires an elegant model to describe device stack and fitting parameters, such as CD, film thickness and real and imaginary refractive indices, n and k. The most common assumption in the model is that of non-variability of the optical properties. However, if the optical properties vary across the wafer, wafer-to-wafer or lot-to-lot, this not-modeled optical variation can impact the CD accuracy and give false alarms. Additionally, as the film stacks become more complex, a larger number of floating parameters in the model are needed. The more parameters floating in the model, the most likely it is to lose CD accuracy and precision due to correlation of floating parameters.
For current scatterometry-based CD metrology, the following problems are apparent: Cross talk of floating parameters in the model; Variation in optical properties due to process stability, such as deposition temperature; No in-die capability, because the size of CD scatterometry target is too large, typically around 40 μm by 40 μm; Long calculation time of the CDs; and It is time consuming to create the scatterometer set-up recipe.
Differential techniques may be used to measure specific parameters of the lithographic process, such as overlay, focus, and lens aberration. Differential techniques help to reduce the burden of recipe creation and allow for targets smaller than the spot size of the scatterometer. Differential techniques require that the differential signal is (close to) zero at the process operating point. This is needed for an effective common mode suppression of signals due to variation of the underlying stack. One use of differential techniques is to design targets that turn asymmetric when the process is deviating from the optimal working point. Target asymmetry can be detected by measuring higher diffraction orders in the scatterometry signal. Examples are overlay and asymmetric focus targets. Another use of differential techniques is to design target pairs, being similar at the optimal working point, but deviating in response to a specific process parameter. An example is aberration sensitive target pairs.
It is a problem that targets are not available for which a difference signal is dominated by the CD variation and that are applicable to the After Develop Inspect and After Etch Inspect steps on the lithographic process.